Method for fabricating bonded-metal articles, particularly tantalium-copper heat exchangers



United States Patent METHGD FOR FABRICATING BONDED-METAL ARTICLEd, PARTICULARLY TANTALIUM-COP- PER HEAT. EXQHANGERS Hugh R. Smith, in, Piedmont, Calif., assignor to Tcmescai Metaiiurgicai Corporation, Richmond, Cahf a corporation of Caiifornia No Drawing. Filed Aug. 26, 1960, Ser. No. 52,035 5 Ciaims. (Ci. 29-527) This invention relates to the fabrication of articles composed of solidly united parts of different materials 5 for example, tubular heatexchangers composed of an inner tubular part of copper solidly united with an outer tubular part of tantalum.

It is well known that many articles are advantageously made by bonding together sheets of dissimilar materials. Nevertheless, there are numerous cases where such structures would be of obvious advantage, but have not been practical heretofore because of difliculty in obtaining a good bond between the materials required. A case in point is heat exchangers used in the chemical industry for heating acids and the like. The best heat exchangers heretofore available for this purpose have been hollow cylinders or tubes of tantalum, which extend into the acid bath and into which hot steam is introduced for heating the acid. These cylinders are made of tantalum because it is the only material available that is satisfactory both in resisting attack by the acid and in having other properties that are acceptable for such a heat exchanger.

The tantalum tubes from which these heat exchangers have been made heretofore must be relatively thick in order to withstand the high pressure of the hot steam. For example, tantalum sheet 60 mils thick has been used. This is not only expensive, bntalso the thick tantalum sheet does not haveidealthermai conduction characteristics, and results inuneven heating.

A much better and cheaper. heat exchanger results if the tantalum is reduced inthickness, say to five mils, and the. necessary mechanicalstrengthobtained from an inner layer of copper. However, it has not been practically heretofore to build such a structure because, under ordinary metal-fabrication conditions, the copper does not form a satisfactory bond to the tantalum. In consequence, the interface between the two metals, as heretofore joined, has been mechanically weak and has had poor thermal conduction.

The present invention solves the above problem. An outer tubular part is formed from thin tantalum sheet and is heated in a high vacuum until its inner surface is thoroughly outgassed and clean. (As herein used, the term high vacuum refers to absolute pressures no greater than approximately one micron of mercury.) In such a vacuum, the surface of the tantalum can be thoroughly outgassed and cleaned by heating to a temperature of approximately 450 centigrade, or higher, for a period of time depending upon the initial purity and condition of the tantalum. While the tantalum part is still in the vacuum chamber and still hot, a cylindrical, graphite rod, or the like, is placed in the center of the tantalum tube to form a core coaxial with the tantalum tube and separated from its inner surface by a tubular, evacuated space. This tubular space is next filled with molten copper, previously melted and outgassed under high vacuum, all the while maintaining the high vacuum to prevent contamination of the hot, clean surface of the tantalum. Under these conditions of absolute cleanliness, obtainable only in high vacua, the molten copper wets the tantalum.

The copper within the tubular space is now cooled and solidified, thereby forming a tubular copper part concentrically disposed within the tubular tantalum part. Because the tantalum surface was wet by the molten cop- 3,120,702 Patented F eb. 11, 1964 2 per under the conditions provided, upon solidification of the copper the two parts are solidly united or 'bonded togetheri.e., the copper and the tantalum are in such intimate contact that they are united by interatomic'bonds into essentially a single solid body. After the copper cools and solidifies, the tubular structure may be withdrawn froin the vacuum chamber and the carbon core bored out or otherwise removed.

The resulting heat exchanger is significantly superior to those heretofore availablein respect to both cost and performance. The lower cost results largely from the saving in tantalum which is a considerably more expensive metal than copper. The copper can be made as thick as desired for adequate mechanical strength, and the excellent thermal characteristics of copper lead to very even heating and otherwise superior performance of the improved heat exchanger. The interface between the copper and the tantalum is mechanically strong, and provides good heat transferbetween the two metals.

Although particularly important in the bonding of copper to tantalum for the manufacture of heat exchangers,

it is evident that the same principles have a Wider application and can be used to bond other materials that may not bond readily under ordinary conditions. Most materials, whether metals or nonmetals, will wet other materials and form strong bonds therewith when the molten material is brought into contact w ith a very clean surface produced by heating in a high vacuum until the surface is thoroughly outgassed. In general, baking out the surface at a temperature of about 450 centigrade, or higher, in a high vacuum, provides the required environment for wetting. Exceptions occur when catastrophic chemical reaction occurs between the two materials when they are brought into contact. For most common metals, this does not occur at the temperatures and pressures necessary to provide clean surfaces, and therefore the methods herein disclosed are of general utility, particularly for solidly uniting one metal part with anoth er metal part.

As a further example, the process and principles herein disclosed have been used to make steel-reinforced castings of aluminum or copper. Specifically, aluminum rotors for rotating machinery may be strengthened by steel inserts at points of high stress, provided the aluminum is solidly united with the steel, which is not accomplished by conventional metal-working techniques. According to the present invention, the steel parts are thoroughly cleaned and outgassed by baking out at about 450 centigrade, or higher, in a high vacuum, and vacuum-melted aluminum is poured and cast in contact with the hot steel while it is still under vacuum. Under these conditions the aluminum wets the steel, and :upon solidification is solidly united therewith. Of course, the process is not limited to rotors: steel-reinforced aluminum castings of any size and shape can be fabricated. Also, copper can be substituted for the aluminum.

What is claimed is:

1. The method for fabricating composite articles composed of united and interatomically bonded dissimilar metals which are non-reactive at the operating conditions which comprises, heating a solid metal at a pressure of not more than about one micron of mercury and at an elevated temperature sufficient to outgas and clean said solid metal at said pressure, continuing said heating until said solid metal is substantially outgassed and cleaned, heating a second dissimilar metal at a pressure of not more than about one micron of mercury and at a temperature sufiicient to provide a molten pool of said second metal at said pressure, continuing said heating of said molten pool until said second metal is substantially outgassed, casting said molten metal in contact with said heated solid metal while maintaining said solid metal and said molten metal at a pressure of not more than about one micron of mercury so that said molten metal wets the surface of said solid metal, and cooling said molten metal, whereby a composite article is provided having an integral interatomic bond between dissimilar metals.

2. The method for fabricating composite articles composed of united and interatomically bonded 'dissimilar metals which are non-reactive at the operating conditions which comprises, heating a solid metal at a pressure of not more than about one micron of mercury and at a temperature of at least about 450 C. until said solid metal is substantially outgassed and cleaned, heating a second dissimilar metal at a pressure of not more than about one micron of mercury and at a temperature suflicient to provide a molten pool of said second metal at said pressure, continuing said heating of said molten pool until said second metal is substantially outgassed, casting said molten metal in contact with said heated solid metal while maintaining said solid metal and said molten metal at a pressure of not more than about one micron of mercury so that said molten metal wets the surface of said solid metal, and cooling said molten metal whereby a composite article is provided having an integral interatomic bond between dissimilar metals.

3. The method for fabricating tubular heat exchangers, which comprises forming an outer tubular part of tantalum sheet, placing said outer part in a high-vacuum chamber evacuated to an absolute pressure no greater than approximately one micron of mercury, heating said outer part to a temperature of at least approximately 450 centigrade until its inner surface is thoroughly outgassed and clean, placing a cylindrical core Within said outer part and separated therefrom by a tubular evacuated space, melting and outgassing a quantity of copper under high vacuum, filling said tubular space with said molten copper, all while maintaining the high vacuum to prevent contamination of the hot, clean, inner surface of said outer part, whereby the molten copper wets said surface, cooling and solidifying the copper within said tubular space, thereby forming a tubular copper part concentric within and solidly united with the tantalum outer part, and remov ing said core.

4. The method for fabricating steel-reinforced aluminum casting which comprises forming a reinforcing part of steel, heating said steel reinforcing part at a pressure of not more than about one micron of mercury and at a temperature of at least about 450 C. until the surface thereof is thoroughly outgassed and cleaned, heating aluminum at a pressure of not more than about one micron of mercury and at a temperature sufficient to provide a molten pool of aluminum at said pressure, continuing said heating of said molten aluminum until said aluminum is substantially outgassed, casting said molten aluminum in contact with said heated steel reinforcing part while maintaining said steel reinforcing part and said molten aluminum at a pressure of not more than one micron of mercury so that said aluminum wets the surface of said steel, and cooling said molten aluminum whereby a steel-reinforced aluminum casting is provided wherein the aluminum and steel are interatomically bonded to one another.

5. The method for fabricating steel-reinforced copper casting which comprises forming a reinforcing part of steel, heating said steel reinforcing part at a pressure of not more than about one micron of mercury and at a temperature of at least about 450 C. until the surface thereof is thoroughly outgassed and cleaned, heating copper at a pressure of not more than about one micron of mercury and at a temperature sufiicient to provide a molten pool of copper at said pressure, continuing said heating of said molten copper until said copper is substantially outgassed, casting said molten copper in contact with said heated steel reinforcing part while maintaining said steel reinforcing part and said molten copper at a pressure of not more than one micron of mercury so that said copper wets the surface of said steel, and cooling said molten copper whereby a steel-reinforced copper casting is provided wherein the copper and steel are interatomically bonded to one another.

References Cited in the file of this patent UNITED STATES PATENTS 232,227 Babbitt Sept. 14, 1880 1,125,159 Page Jan. 19, 1915 1,943,853 Austin Jan. 16, 1934 2,117,722 Huggins May 17, 1938 2,176,773 Sparkes Oct. 17, 1939 2,713,196 Brown July 19, 1955 

1. THE METHOD FOR FABRICATING COMPOSITE ARTICLES COMPOSED OF UNITED AND INTERATOMICALLY BONDED DISSIMILAR METALS WHICH ARE NON-REACTIVE AT THE OPERATING CONDITIONS WHICH COMPRISES, HEATING A SOLID METAL AT A PRESSURE OF NOT MORE THAN ABOUT ONE MICRON OF MERCURY AND AT AN ELEVATED TEMPERATURE SUFFICIENT TO OUTGAS AND CLEAN SAID SOLID METAL AT SAID PRESSURE, CONTINUING SID HEATING UNTIL SAID SOLID METAL IS SUBSTANTIALLY OUTGASSED AND CLEANED, HEATING A SECOND DISSIMILAR METAL AT A PRESSURE OF NOT MORE THAN ABOUT ONE MICRON OF MERCURY AND AT A TEMPERATURE SUFFICIENT TO PROVIDE A MOLTEN POOL OF SAID SECOND METAL AT SAID PRESSURE, CONTINUING SAID HEATING OF SAID MOLTEN POOL UNTIL SAID SECOND METAL IS SUBSTANTIALLY OUTGASSED, CASTING SAID MOLTEN METAL IN CONTACT WITH SAID HEATED SOLID METAL WHILE MAINTAINING SAID SOLID METAL AND SAID MOLTEN METAL AT A PRESSURE OF NOT MORE THAN ABOUT ONE MICRON OF MERCURY SO THAT SAID MOLTEN METAL WETS THE SURFACE OF SAID SOLID METAL, AND COOLING SAID MOLTEN METAL, WHEREBY A COMPOSITE ARTICLE IS PROVIDED HAVING AN INTEGRAL INTERATOMIC BOND BETWEEN DISSISMILAR METALS. 